ROME—Pope Francis is suffering from the first signs of pneumonia, the Vatican said on Tuesday, signaling a potentially lengthy stay in the hospital as concerns about the health of the 88-year-old pontiff grow.

In a brief note to journalists Feb. 18, the Vatican said, "Due to the Holy Father's health condition," his appointments had been canceled through Feb. 22.

Pope Francis, 88, was hospitalized last Friday with bronchitis, the Vatican said, and now has a “polymicrobial infection of the respiratory tract.”